1. Field of the Invention
The present invention relates to high purity cobalt in which a content of impurities such as copper is reduced, a method of manufacturing thereof, and high purity cobalt targets.
2. Description of the Related Art
Semiconductor devices such as VLSI (very large scale integrated circuit) and ULSI (ultra LSI), have a structure where various thin metal films are deposited on, for example, a silicon (Si) wafer. Although the idea of using cobalt (Co) as a wiring material has been considered in recent years, the accompanying injurious impurities in the cobalt may result in malfunction or deterioration of the semiconductor device, which is undesirable. For example, copper (Cu) may cause a short circuit because of high diffusion rate inside silicon, and radioactive elements such as uranium (U) and thorium (Th) will cause incorrect operations, and alkaline metals and alkaline-earth metals may cause degradation of the device properties.
While levels in purity of the crude cobalt traded globally and presently are about 98% to 99.8%, such crude cobalt contains various impurities, for example, transition metals such as nickel (Ni) iron (Fe), and chromium (Cr), gas elements such as oxygen (O), nitrogen (N), and sulfur (S). Therefore, in order to use cobalt for a semiconductor device, it is necessary to remove these impurities from the crude cobalt and achieve higher purification. Moreover, cobalt appears favorable as material of devices such as magnetic recording mediums or magnetic recording heads, as well as semiconductor devices, because of bearing properties typical of ferromagnetic metals. A higher purification of cobalt is indispensable to the use of cobalt as material of these devices.
Various methods of removing impurities from crude cobalt, for example, wet processing such as solvent extraction, ion exchange, and electrolytic refining for separation of metallic elements, and dryness hydrogen gas (H2) processing for removal of gas elements such as oxygen and nitrogen, and floating zone melting refining method, have been studied.